I`tikaf (spiritual retreat in the mosque) during the last ten days of Ramadan in the Masjid is a beautiful Sunnah of the Prophet (peace be upon him).  `A’ishah (may Allah be pleased with her) reported that the Prophet (peace be upon him) used to observe i`tikaf during the last ten days of the month of Ramadan and he continued this practice until his death. Following him, his wives carried on this practice of observing i`tikaf regularly. (Al-Bukhari and Muslim)

It is recommended that every Muslim make i`tikaf at least one time in his or her life. There must be at least few people in every community who should perform it in the Masjids every year to keep this Sunnah alive.

Women can also do i`tikaf, but theirs must be at home for reasons of modesty and decency. There is a great blessing in i`tikaf and people who do it bring blessings upon themselves, upon their families, and upon the whole Muslim community.

It is recommendable to start i`tikaf on the evening of the 20th of Ramadan and continue until the dawn of the day of `Eid Al-Fitr. During i`tikaf men must stay in the Masjid all the time, and they are not allowed to leave it except for a necessity.

During i`tikaf one should spend one’s time in prayer, reading the Qur’an, dhikr (remembrance of Allah) and reflection. Engaging in business and other worldly conversation and activities is not allowed during i`tikaf. However, attending a Janazah (Funeral), visiting a sick person or leaving for any emergency is permissible.

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Source: onislam.net